February 18, 2026
Vintage drama alert!
Instruction decoding in the Intel 8087 floating-point chip
He cracked open a ’80s math chip—and then jumped into the comments
TLDR: Ken Shirriff tore down the 1980s Intel 8087 math chip and explained how it decodes instructions by watching the CPU for special signals. The headline moment is the author jumping into the comments to take questions, igniting nostalgia and debate over whether the 8087 was genius upgrade or pricey novelty.
Ken Shirriff just did the nerd equivalent of a heist movie: he literally cracked open an Intel 8087—the add‑on math chip that made 1980s PCs fly—and showed how it secretly “listens” for special codes from the main CPU to run fancy math like π and logarithms. The kicker? The author himself popped into the thread, writing, “Author here for all your 8087 questions…”, and the vibe instantly turned into a surprise AMA.
Fans are swooning over the microscope‑glam die shots and the wild idea that the 8087 acted like a math cop, eavesdropping on the bus for “escape” codes. The big debate brewing: was the 8087 a brilliant upgrade that powered spreadsheets and CAD, or an expensive flex only a few pros ever needed? There’s also chatter about the 8087’s quirky stack of registers—some call it elegant, others say it was a mind‑bender that haunted programmers. Meanwhile, the microcode “brain” running the instructions feels like retro sci‑fi to newcomers.
Humor is bubbling up around the idea of a chip that “spies” on the CPU—cue the “undercover math cop” memes—and the perennial joke that the 8087 was just a pricey way to get faster “divide by zero” errors. But make no mistake: with the author in the comments, the community wants answers and stories, and they’re treating this like a vintage‑tech red carpet moment. Read the post here.
Key Points
- •The Intel 8087 added 62 floating‑point instructions to the IBM PC platform, including transcendental functions and constants like π.
- •Instruction decoding within the 8087 is distributed across multiple circuits; the article explains these mechanisms via die analysis.
- •The 8087 die features a bottom‑half datapath for 80‑bit floating point, a constant ROM, and eight floating‑point registers organized as a stack.
- •Instruction behavior is defined by a central microcode ROM, with a microcode engine executing the corresponding micro‑instructions.
- •Cooperation with the 8086/8088 relies on eight ESCAPE opcodes detected by the 8087’s BIU; data exchange occurs through memory due to lack of shared registers.